DO SILICIFICATION EXPERIMENTS MIMIC NATURAL CONDITIONS? A COMPARISON OF SILICA SPECIATION IN EXPERIMENTAL SOLUTIONS WITH THOSE OF NATURAL SYSTEMS

Silica-rich hot springs are targets for taphonomic studies because of their mineralizing solutions, thermal constancy, ancient microbial lineages, and microbial diversity. The manner in which microorganisms become incorporated into the geologic record affects the fidelity of the resulting microfossils. Solution chemistry, namely speciation and degree of saturation, is critical in controlling the timing of the fossilization process. The question remains regarding whether silicification experiments emulate natural conditions, and how the differences affect the fidelity of silicification. Herein we present analytical and modeled solution compositions for two hot springs in Yellowstone National Park, WY: Queen's Laundry and Ojo Caliente. The compositions of these springs were modeled using EQ3NR/EQ6, and compared with published experimental solution compositions. For Queen's Laundry, silica is present mainly as aqueous silica (SiO2(aq)), also known as silicic acid (H4SiO4 o), with the second most abundant species H3SiO4-. Silica polymers are calculated to be present at very low concentration. Under all reported experimental conditions modeled, aqueous silica (SiO2(aq)) is the dominant species. Interestingly, though, the minor silica species are not distributed the same order for all experiments. Dissociated silicic acid (H3SiO4-), neutral sodium silicate ion (NaH3SiO4o), and silica polymers dominate the percentage of silicon not in silicic acid, depending on the experimental conditions. The speciation is important because dissociated silicic acid or negatively charged polymers are repulsed by negatively charged bacterial surfaces while silicic acid and other neutrally charged species are not repulsed, although they are not strongly attracted to such surfaces either. Silica saturation is also an important factor in deposition. Published silicification experiments used dissolved silica concentrations 10 or more times oversaturated with respect to amorphous silica, while natural concentrations are near the saturation level. Higher degrees of saturation correspond to a greater degree of polymerization, and higher deposition rates.